Very small sterling screwback pin. I cannot say for sure but I think this could signify being one of the first 500 Shriners ever, a level reached in the late 1870's. Ten years later there were over 7,000 members and it would not surprise me if this pin dated from one of the annual conventions around that time. Red and white enamel applied.

Tricky to photograph, may try a scanner as well when I can.